The anaphase-promoting complex or cyclosome (APC/C) is the ubiquitin ligase that regulates mitosis by targeting specific proteins for degradation at specific times under the control of the spindle assembly checkpoint (SAC). How the APC/C recognizes its different substrates is a key problem in the control of cell division. Here, we have identified the ABBA motif in cyclin A, BUBR1, BUB1, and Acm1, and we show that it binds to the APC/C coactivator CDC20. The ABBA motif in cyclin A is required for its proper degradation in prometaphase through competing with BUBR1 for the same site on CDC20. Moreover, the ABBA motifs in BUBR1 and BUB1 are necessary for the SAC to work at full strength and to recruit CDC20 to kinetochores. Thus, we have identified a conserved motif integral to the proper control of mitosis that connects APC/C substrate recognition with the SAC. •The conserved ABBA motif that binds to APC/C coactivators is identified•The ABBA motifs of cyclin A and BUBR1 bind the same site on CDC20•The ABBA motif is required for rapid cyclin A degradation in prometaphase•The ABBA motifs in BUBR1 and BUB1 are required for a fully functional SAC Di Fiore et al. identify an interaction motif, “ABBA,” common to both BubR1, a spindle assembly checkpoint (SAC) protein, and cyclin A, a mitotic regulator. This motif mediates binding of these proteins to the APC/C activator Cdc20 and is required for SAC function and timely cyclin A degradation.